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CHAPTER 14
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G L O S S A R Y
A
Aberration Deviation from normal devel-
opment or growth; a lesion or anomaly.
Absolute risk Model predicting that
a specific number of excess cancers will
occur as a result of exposure to ionizing
radiation.
Absorbed dose (D) The amount of
energy per unit mass absorbed by an
irradiated object (e.g., the patient’s body
tissue). This absorbed energy is responsible for any biologic damage resulting
from the tissues being exposed to radiation. The gray (Gy) is the SI unit of this
radiation quantity.
Acids Hydrogen-containing compounds
that can attack and dissolve metal (e.g.,
HNO3, nitric acid).
Action limits Limits for occupational
exposure that are set by the medical facility well below the regulatory values as
they appear in state or federal regulations.
These limits are set at levels, typically a
tenth of the action limit, that are not routinely exceeded by personnel. They are
meant to trigger an investigation that
should uncover the reason for any unusually high exposure.
Acute Something that begins suddenly
and runs a short but severe course (e.g.,
an acute disease).
Acute radiation syndrome (ARS) Radi-
ation sickness, or early somatic tissue
reactions, occurring in humans soon after
whole-body reception of large doses of
ionizing radiation delivered over a short
period.
Added filtration Sheets of aluminum (or
its equivalent) of appropriate thickness
interposed outside the glass window of
the x-ray tube housing above the collimator shutters.
Adenine (A) One of two purine bases
found in both DNA and RNA.
Agreement states Individual states of the
United States that have entered into an
agreement with the Nuclear Regulatory
Commission (NRC) to assume responsibility for enforcing radiation protection
regulations through their respective health
departments.
Air gap technique An alternative proce-
dure to the use of a radiographic grid for
reducing scattered radiation during certain examinations.
Air kerma SI quantity that can be used to
express radiation energy transferred to a
point, such as the surface of a patient’s or
radiographer’s body. Air kerma is kinetic
energy released in a unit mass (kilogram)
of air and is expressed in metric units of
joule per kilogram (J/kg).
ALARA concept/principle Precept hold-
ing that occupational exposure of the patient, occupationally exposed persons,
and the general public should be kept “as
low as reasonably achievable.” Radiation
exposure should always be kept ALARA
for all medical imaging procedures.
Alkali A member of a group of elements
that includes lithium, sodium, and potassium.
Alkaline earth A member of a group of
elements including calcium, magnesium,
and strontium.
Alliance for Radiation Safety in Pediat-
ric Imaging A partnership of medical
societies, founded in 2007, whose overall
common purpose is to reduce dose for
pediatric patients.
Alpha particle A positively charged par-
ticle of radiation that is emitted from
nuclei of very heavy elements such as
uranium and plutonium during the process of radioactive decay. An alpha particle contains two protons and two neutrons and therefore carries an electric
charge of plus two.
Aluminum oxide (Al2O3) Sensing mate-
rial found in optically stimulated luminescence dosimeters.
American Association of Physicists in
Medicine (AAPM) Professional orga-
nization that is the primary scientific and
educational body for medical physicists
and is also responsible for accrediting
laboratories that calibrate instruments
used to measure radiation exposure in
medical radiology.
American College of Radiology (ACR)
Major professional organization of radiologists in the United States.
American Registry of Radiologic Tech-
nologists (ARRT) Nongovernmental
credentialing organization that tests and
certifies radiologic technologists on a national level. The intention is to “seek and
ensure quality patient care in radiologic
technology.”
American Society of Radiologic Tech-
nologists (ASRT) The “premier pro-
fessional association” of persons employed in medical imaging and radiation
therapy. It provides education, advocacy,
and research for the membership.
Amino acids The structural units of protein.
Amorphous selenium A noncrystalline
grouping of silicon atoms in which, rather
than a regular geometric pattern, the silicon atoms are distributed in a continuous
random fashion.
Ampere The SI unit of electrical charge.
One ampere represents the quantity of
electrons amounting to a charge of 1 coulomb crossing unit area per second.
Analog image A nondigital visible image
produced by x-radiation on a developed
radiographic film or other viewing media.
Anaphase The phase of mitosis during
which the duplicate centromeres migrate
in opposite directions along the mitotic
spindle and carry the chromatids to opposite sides of the cell.
Anemia A condition characterized by a
lack of vitality and caused by a decrease in
the number of red blood cells in the circulating blood.
Anion A negatively charged ion.
Annihilation radiation Radiation in the
form of two oppositely moving 511 keV
photons generated as the result of the
mutual annihilation of matter and antimatter (i.e., an electron and a positron).
Annual occupational effective dose
(EfD) limit An upper boundary limit
for radiation workers for yearly wholebody exposure (excluding personal medical and natural background exposure) of
50 millisieverts (mSv). There is also an
added recommendation that the lifetime
EfD in mSv should not exceed 10 times
the occupationally exposed person’s age
in years.
370
GLOSSARY
371
Anode The positively charged target in the
x-ray tube.
Antibodies Materials developed by the
body in response to the presence of
foreign bodies, called antigens, such as
bacteria or a virus. Once the skin is penetrated, they provide a primary defense
mechanism against antigens.
Antigens Any substance that causes an
immune system to produce antibodies
against it; usually a foreign substance,
such as a toxin or a component of a virus,
bacterium, or parasite.
Antimatter Matter composed of the
counterparts of ordinary matter that does
not exist freely in the universe and is unstable in the presence of ordinary matter.
Aplastic anemia Anemia resulting from
bone marrow failure.
Apoptosis A nonmitotic or nondivision
form of cell death that occurs when cells
die without attempting division during
the interphase portion of the cell life cycle. Also known as programmed cell death
(formerly called interphase death).
Artifact A structure or an appearance that
is not normally present on a radiograph
and is produced by artificial means.
Artificial radiation See human-made
radiation.
Ataxia An inability to coordinate volun-
tary muscular movements.
Atom The smallest portion of an element
that has all of its chemical properties.
Atomic number The number of protons
(Z) contained within the nucleus of an
atom.
ATP (adenosine triphosphate) A type of
molecule found in every cell. Its function
is to store and supply the cell with energy.
Atrophy A shrinkage of any body part
that may follow substantial partial-body
radiation exposure.
Auger effect When an inner-shell va-
cancy occurs in an atom, the energy liberated when this vacancy is filled can be
transferred to another electron of the
atom, thereby ejecting the electron. The
process is known as the Auger effect, and
the emitted electron is known as an Auger
electron.
Automatic collimation See Positive beam
limitation (PBL).
Automatic Exposure Control (AEC) A
system used to consistently control the
amount of radiation reaching an image receptor by terminating the length of exposure
when a satisfactory precalibrated level of
exposure is registered by one or more detectors. When using AEC systems, the radiographer generally must still use individual experience and discretion to select
an appropriate kVp, mA, image receptor,
and grid.
Axial CT scan A type of CT scan which is
a result of an in-plane step and shoot sequence of the scanner table and the
x-ray respectively. For this the pitch is
equal to one.
Axon A long, single tentacle from the nerve
cell body that carries impulses away from it.
B
Background equivalent radiation time
(BERT) Method to compare the amount
of radiation received from a radiologic
procedure with natural background radiation received over a specified period
such as days, weeks, months, or years.
Backscatter Photons that have interacted
with the atoms of an object and as a result
are deflected backward (toward the x-ray
tube).
Bases In general, any substance that in
water solution is slippery to the touch,
tastes bitter, reacts with acids to form
salts, and promotes certain chemical reactions. In chemistry, a base is a chemical
species that donates electrons, accepts
protons, or releases hydroxide (OH-) ions
in aqueous solution. In the cell, bases
(adenine, thymine, guanine, and cytosine
or uracil) serve as the major compounds
of DNA and RNA forming the rungs of
the ladder present in these cellular nuclear macromolecules.
Beam direction factor See Use factor (U).
Becquerel The SI unit of radioactivity. It
is equal to 1 disintegration (decay) per
second.
Beta decay The process wherein a nu-
cleus relieves instability by a neutron
transforming itself into a combination of
a proton and an energetic electron (called
a beta particle). There is also emission of
another particle called a neutrino, which
has negligible mass and no electric charge
but carries away any excess energy.
Beta particles High-speed electrons ejected
from a nucleus that undergoes beta decay.
They are also known as beta rays.
Binding energy Force that holds the
components of an atom or a nucleus together.
Biologic dosimetry A method of dose as-
sessment in which biologic markers or effects of radiation exposure are measured
and the dose to the organism is inferred
from previously established dose–effect relationships. Examples include white blood
cell counts and chromosomal aberrations.
Blebs Tiny membrane-enclosed structures
that are produced when cells shrink in
apoptosis.
Blood plasma Blood plasma is a fluid
component of blood that contains in suspension the blood cells of whole blood. It
is essentially the agent that carries cells
and proteins throughout the body. It
makes up about 55% of the body’s total
blood volume.
Bone marrow syndrome See Hemato-
poietic syndrome.
Brachytherapy The treatment of cancer
by the insertion of radioactive materials
or implants either temporarily or permanently directly into the cancerous tissue
and its immediate vicinity.
Bragg–Gray theory Relates the ioniza-
tion produced in a small cavity within an
irradiated medium or object to the energy
absorbed in that medium as a result of its
radiation exposure.
Bremsstrahlung Polyenergetic ionizing
electromagnetic radiation that is produced
when a beam of electrons in an x-ray tube
undergoes deceleration by interaction with
the nuclei of the x-ray tube target atoms.
Broad beam x-ray transmission factor
(B) The ratio of air kerma (K
) behind a
a
barrier of material thickness “x” to the
value of Ka at the same location with no
intervening barrier.
Bucky grid An assembly of moving lead
strips resembling a venetian blind, placed
between a patient being x-rayed and the
image receptor to improve the collimation and reduce scatter radiation on the
detector.
Bucky slot shielding device A protective
device made of at least 0.25-mm lead
equivalent that automatically covers the
Bucky slot opening in the side of the x-ray
table during a fluoroscopic examination
when the Bucky tray is positioned at the
foot end of the table. This protects the
radiographer and the radiologist from
gonadal radiation exposure.
Bureau of Radiological Health (BRH) See
Center for Devices and Radiological Health
(CDRH).
372
GLOSSARY
C
Calibrated instrument Any device that
is compared with a generally accepted
(nationally or internationally) standard
device so that its accuracy is determined.
Radiation survey instruments are usually
calibrated annually.
Candela SI unit used to describe the degree
of luminance. One candela corresponds to
3.8 million billion photons per second being emitted from a light source through a
cone-like field of view. A common wax
candle emits light with a luminous intensity of roughly one candela.
Carbohydrates Compounds, also known
as saccharides, composed of only carbon,
hydrogen, and oxygen. Carbohydrates
such as sugars and starches are involved in
energy-releasing processes in animals and
plants.
Carbon Nonmetallic element that is the
basic constituent of all organic matter.
Carbon fiber Material used in the tops of
radiographic tables. It has a lower x-ray
absorption potential when compared
with materials such as aluminum but has
the strength to support adult patients.
The use of carbon fiber results in lower
radiographic techniques for producing
the recorded image, thereby lowering patient dose.
Carcinogenesis The production or origin
of cancer.
C-arm fluorosc ope A portable device
for producing real-time images of areas
of patient anatomy. The opposite ends
of the C-shaped support arm are respectively the x-ray tube and the image
receptor.
Catabolism The breaking down in living
organisms of more complex molecules into
simpler ones, with the release of energy.
Catalyst Agent that affects the speed of a
chemical reaction without being altered
itself.
Catalytic failure The inability to influence
the speed of a required chemical reaction
(e.g., during protein synthesis).
Cataract Opacity of the lens of the eye.
Cataractogenesis The production or ori-
gin of cataracts.
Cathode A negative electrode. The cathode
is the source of the high-speed electrons in
an x-ray tube.
Cation A positively charged ion.
Cell division The process whereby one
cell divides to form two or more cells.
Cell membrane The frail, semipermeable,
flexible structure encasing and surrounding the human cell. It functions as a barricade to protect cellular contents from
their outside environment and controls
the passage of water and other materials
into and out of the cell.
Cell metabolism The series of chemical re-
actions that modifies foods for cellular use.
Cell survival curve A graphical method
of displaying the sensitivity of a particular type of cell to lethal effects of
radiation.
Cells The basic units of all living matter.
Cellular life cycle The passage of a cell
through the phases G1, S, G2, and M.
Center for Devices and Radiological
Health (CDRH) Known before 1982 as
the Bureau of Radiological Health (BRH),
this agency is responsible for conducting
an ongoing electronic product radiation
control program.
Centigray (cGy) One one-hundredth of a
gray (1/100 Gy), previously known as a rad.
Centrioles A pair of small, hollow, cylin-
drical structures oriented at right angles
to each other and embedded in a material
mass of more than 100 proteins. They
play a significant role in the formation of
the mitotic spindle during cell division.
Centromere A clear region on a chromo-
some serving as a joining point; it is actually the center of the chromosome.
Centrosomes Structures, located in the
center of the cell near the nucleus at each
end of the mitotic spindle, that contain
the centrioles.
Cerebrovascular syndrome Form of
acute radiation syndrome that results
when the central nervous system and cardiovascular system receive doses of 50 Gyt
or more of ionizing radiation. A dose of
this magnitude can cause death within a
few hours to 2 or 3 days after exposure.
Characteristic photon A quantum or
quantity of radiant energy given off by an
atom when an electron from an outershell drops down to fill an inner-shell
vacancy. The energy of a characteristic
photon is equivalent to the difference in
energy level between the two electron
shells. Also known as characteristic x-ray
or fluorescent radiation.
Characteristic radiation Radiation con-
sisting of characteristic photons. In a
general-purpose x-ray tube characteristic
radiation comprises about 10% of the
primary radiation beam for tube voltages
between 80 and 100 kVp.
Charge-coupled device (CCD) A device
that, when struck by visible light, produces electrical signals in proportion to
the brightness of the light. CCDs are used
in indirect types of digital x-ray detectors.
Indirect digital detectors use a phosphor
to convert the x-ray energy to visible light,
after which the CCD converts the visible
light into electrical signals.
Chromatid A highly coiled strand; one of
the two duplicate portions of DNA in a
replicated chromosome that appear during cell division.
Chromatid aberrations Deviation from
normal development or growth. Lesions
that result when irradiation of individual
chromatids occurs later in interphase, after DNA synthesis has taken place.
Chromatin The substance distributed in
the nucleus of a cell that condenses to
form chromosomes during cell division.
Chromosome aberrations Deviation from
normal development or growth. Lesions
that result when irradiation occurs early
in interphase, before DNA synthesis takes
place.
Chromosome breakage The segmenting
of a chromosome due to the breaking of
one or both of the sugar–phosphate chains
of a DNA ladder-like structure, which is a
potential outcome when ionizing radiation
interacts with a DNA macromolecule.
Chromosomes Tiny, rod-shaped bodies
that contain genes.
Chronic Something that continues for a
long time (i.e., a chronic disease).
Classical scattering See Coherent scat-
tering.
Clear lead shields Transparent lead–
acrylic material that has been impregnated with approximately 30% lead by
weight. It is used for viewing windows
and pull-down or roll-away x-ray room
shielding.
Cleaved chromosome A broken chro-
mosome.
Code of Standards for Diagnostic X-Ray
Equipment Effective August 1, 1974,
this code applies to complete x-ray systems and their major components manufactured after that date.
Coherent scattering The process wherein
a low-energy photon (typically less than
10 keV) interacts with an atom of human
tissue and does not lose its energy. The
GLOSSARY
373
atom responds by releasing the energy it
has received in the form of a scattered
photon that has the same wavelength and
energy as the original incident photon.
The emitted photon changes direction by
20 degrees or less. No ionization of the
biologic atom occurs. Also known as clas-
sical scattering, elastic scattering, unmodified scattering, and Rayleigh scattering.
Thompson scattering, which involves a free
electron and not an atom, is another type
of coherent scattering.
Collective effective dose (ColEfD) Des-
ignated for use in the description of a
population or group exposed to different
individual amounts of ionizing radiation.
It is equal to the sum of all of the doses
times the number of individuals exposed
and would be expressed in units such as
person-sievert.
Collimation Limiting all the margins of
the useful x-ray field to a specific size and
shape to confine the beam to the anatomic area of interest. This reduces the
dose that a patient receives and improves
contrast of the radiographic image.
Committed effective dose equivalent
(CEDE) A measure of the probabilistic
health effect on an individual as a result
of intake of radioactive material into the
body. It takes into account the length of
time that the radioactive material may
stay within the body.
Compensating filter A material such as
aluminum, lead–acrylic, or other suitable
material inserted between the x-ray source
and the patient to modify the quality
(penetrating power, spectrum) of the
beam across the field of view.
Compton scattered electron An ener-
getic electron dislodged from the outer
shell of an atom of the irradiated object as
a result of a Compton interaction with an
incoming x-ray photon. Also known as a
secondary, or recoil, electron.
Compton scattered photon X-ray pho-
ton of weaker energy emerging from an
atom, usually in a new direction, following
a Compton interaction with that atom.
Compton scattering An interaction be-
tween an incoming x-ray photon and a
loosely bound outer-shell electron of an
atom in the irradiated object. The photon
surrenders a portion of its energy to dislodge the electron from its outer-shell orbit,
thereby ionizing the atom, and then continues in a new direction. This process
accounts for most of the scattered radiation
produced during diagnostic procedures.
Also known as incoherent scattering, inelas-
tic scattering, or modified scattering.
Computed axial tomography (CAT) See
Computed tomography (CT).
Computed radiography (CR) Process in
which an image is captured on a removable digital storage cassette, using storage
phosphor technology. The cassette is then
taken to a “reader” that interprets the
stored signal and transfers it in a digital
matrix to a picture archiving and communication system (PACS) system. CR
has been largely replaced by digital radiography, or DR.
Computed tomography (CT) “The pro-
cess of creating a cross-sectional tomographic plane of any part of the body.”
This computer-reconstructed image of a
patient is created by “an x-ray tube and
detector assembly rotating 360 degrees
about a specified area of the body.” CT has
also been referred to as computerized axial
tomography (CAT).
Congenital abnormalities Defects exist-
ing at birth that are not inherited but rather
acquired during development in utero.
Console See Control panel.
Consumer-Patient Radiation Health and
Safety Act of 1981 Provides federal
legislation requiring the establishment of
minimal standards for accreditation of
educational programs for persons who
perform radiologic procedures and the
certification of such persons.
Contrast media (negative) The use of
air or gas to enhance visualization of
body structures during a radiologic procedure.
Contrast media (positive) A liquid solu-
tion containing an element with a higher
atomic number than surrounding tissue
(e.g., barium or iodine) that is either ingested or injected into biologic tissues or
structures to be visualized.
Contrast Resolution A measure of the
ability to distinguish small x-ray attenuation differences in a CT image.
Control-booth barrier A permanent se-
cured protective barrier for imaging personnel that is located in an x-ray room
that contains housing for permanent or
nonportable radiographic equipment. It
may be regarded as a secondary protective
barrier since the primary x-ray beam is
never directed toward it.
Control monitor Dosimeter provided by
the monitoring company with each batch
of dosimeters to serve as a basis for comparison with the remainder of the dosimeters after they have been returned to the
monitoring company for processing. The
control monitor determines whether the
batch of dosimeters has been exposed to
radiation in transit to or from the health
care facility.
Control panel Site where technical expo-
sure factors such as milliamperes (mA)
and peak kilovoltage (kVp) are selected
and visually displayed. Also called a con-
sole. Besides having buttons for initiating
the exposure, it indicates the conditions
of exposure and shows when the x-ray
tube is energized.
Controlled area A region that is occupied
only by occupationally exposed personnel
and others under their direct supervision.
Coronary heart disease (CHF) Malady
caused by various anatomical changes or
defects that adversely affect the usual
function of the heart.
Cosmic radiation (cosmic rays) Very-high-
energy particles and photons. They result
from immense energy emitting processes
that have taken place throughout the universe, including the sun and other stars in
this galaxy.
Coulomb (C) Basic SI unit of electrical
charge. It represents the quantity of electrical charge flowing past a point in a circuit
in 1 second when an electrical current of
1 ampere is used.
Coulomb per kilogram (C/kg) SI unit of
radiation exposure: 1 coulomb per kilogram (C/kg) of air equals 1 SI unit of exposure, or 1/(2.58 3 1024) 5 3.88 3 103
R. C/kg (traditional unit: roentgen). Both
of these units can be used for x-ray equipment calibration.
Covalent bond A chemical union or cross-
link created between atoms by the sharing
or transfer of one or more electrons.
Crookes tube Partially evacuated electrical
discharge tube used by Wilhelm Conrad
Roentgen when he discovered x-rays.
Crossover Process occurring during mei-
osis in which the chromatids exchange
some chromosomal material (genes).
CT Angiography (CTA) A CT examina-
tion of the chest that is used to diagnose
and evaluate blood vessel disease or related conditions, such as ruptures or narrowing or blockage. Performed either
374
GLOSSARY
with or without the insertion of contrast
material.
CTDI‚ CT dose index (CTDI) is a standard-
ized measure of radiation dose output of a
CT scanner that allows the user to characterize the radiation output of CT scanners
for various protocol technique settings. It
refers to the dose to a standard 16-cm
(head) or 32-cm (body) plastic phantom
as measured by an inserted 10 cm long
pencil-like ionization chamber.
Cumulative effective dose (CumEfD)
limit A radiation worker’s lifetime EfD
must be limited to their age in years
times 10 mSv. This limit pertains to the
whole body.
Cumulative timing device A required
resettable device on a fluoroscopic x-ray
unit that measures the elapsed x-ray
beam-on time and sounds an audible
alarm after the fluoroscope has been activated for 5 minutes.
Curie The standard unit of radioactivity in
use before the SI system of units was
established. One curie is defined as being
equal to 3.7 3 1010 nuclear disintegrations per second (dps).
Cutie pie Nickname for an ionization
chamber–type survey meter.
Cyclotrons Units that, with the aid of a
strong static magnetic field and rapidly
oscillating electric fields, produce in a
circular evacuated cavity beams of highenergy charged particles such as 150-MeV
proton beams.
Cytogenetics The study of cell genetics
with emphasis on analysis of chromosomes.
Cytoplasm The protoplasm that exists
outside of the cell’s nucleus.
Cytoplasmic organelles Miniature cellu-
lar components present in the cytoplasm
that enable the cell to perform normal
functions.
Cytosine (C) One of two pyrimidine bases
found in both DNA and RNA.
D
Daughter cell A cell resulting from the di-
vision of a prior cell (called a parent cell).
Dead-man–type fluoroscopic exposure
switch A fluoroscopic exposure switch
(operated by foot pressure) that requires
continuous pressure from the operator. If
the operator becomes incapacitated, the
exposure automatically terminates.
Deep equivalent dose External whole-
body absorbed dose determined by a
personnel dosimeter at a tissue depth of
1 cm (1000 mg/cm2).
Deletion When a part of the chromosome
or chromatid is lost at the next cell division, thus creating an aberration known
as an acentric fragment. This will result in
a cell mutation.
Dendrites Tentacle like or short branched
extensions of a nerve cell (neuron), along
which impulses or electrical signals received from other cells are transmitted to
the cell body.
Deoxyribonucleic acid (DNA) A type of
nucleic acid (very large biomolecules)
that carries the genetic information necessary for cell replication and regulation
of cellular activity needed to direct protein synthesis. It is often referred to as the
master chemical in the cell because it contains all the information that the cell
needs to function and reproduce.
Deoxyribose A five-carbon sugar mole-
cule (C5H10O4), derived from the sugar
ribose by the loss of one oxygen atom,
that helps form the phosphate backbone
of DNA molecules.
Dermis Middle layer of skin composed of
connective tissue.
Desquamation Shedding or flaking off of
the outer layer of skin; can occur after
high fluoroscopic radiation doses.
Detector configuration In CT scanning,
detector configuration refers to the number of data channels (i.e., detector rows)
being used in the z-axis direction and the
effective detector thickness of each data
channel. For example, a detector configuration described as 128 3 0.5 mm would
signify the use of 128 data channels along
the z-axis, each of which has an effective
thickness of 0.5 mm.
Diagnostic efficacy The degree to which a
diagnostic study accurately reveals the presence or absence of disease in the patient.
Diagnostic-type protective tube hous-
ing The lead-lined metal housing
enclosing the x-ray tube that protects
both the radiographer and the patient
from leakage radiation by predominantly
restricting the emission of the x-rays to
the area of the useful, or primary, beam.
Dicentric chromosomes Chromosomes
that have two centromeres.
Diffusion The motion of liquid, gas, or
solid particles from an area of relatively
high concentration to an area of lower
concentration.
Digital breast tomography (DBT) An
imaging technique which involves multiple
brief radiation exposures occurring while
the x-ray tube rotates within a limited arc
about a patient’s stationary compressed
breast. It is often called 3D mammography
because this tomographic image acquisition method adds a third dimension
(z-axis) that provides depth resolution to
the breast image.
Digital breast tomosynthesis The pro-
cess whereby multiple planar images or
image slices of the breast are reconstructed or synthesized from a series of
mammographic exposures taken at multiple gantry angles relative to the plane of
the compressed breast.
Digital fluoroscopy A technique in which
the fluoroscopic image is produced in
digital form in the image acquisition process. In nondigital fluoroscopy, a standard
image intensifier is employed whose output is transmitted to an analog television
camera which displays the image.
Digital image An image, generated by a
computer, composed of picture elements
or pixels, each with finite, discrete quantities of numeric information for its intensity or brightness.
Digital radiography The use of a flat
panel detector to record a radiographic
image and render it in digital form without a film developing process or the scanning of the image receptor.
Digital subtraction angiography A fluo-
roscopic technique used in interventional
radiology for visualizing blood vessels.
Radiopaque structures such as bone are
eliminated by digitally subtracting them
using an initial reference image or mask
from subsequent images, thus allowing for
isolated depiction of the blood vessels.
Direct action Biologic damage that occurs
as a non-secondary process of ionization
of atoms on essential molecules, which
may cause these molecules to become
either inactive or functionally altered.
Direct radiation See Primary radiation.
Direct transmission Primary x-ray pho-
tons that traverse a patient without interacting.
Dirty bomb See Radioactive dispersal device.
Distance A very important method of protec-
tion from ionizing radiation. Imaging personnel receive significantly less radiation
exposure by standing farther away from a
source of radiation because an inverse
GLOSSARY
375
square law governs the decrease in the radiation level with increased distance.
DNA synthesis The building up of DNA
macromolecules.
Dominant mutation A genetic mutation
that will probably be expressed in offspring. A genetic trait is considered domi-
nant if it is expressed in a person who has
only one copy of that gene as opposed to
a recessive trait which will be expressed in
offspring only when two copies of the recessive gene are present.
Doppler shift Refers to the apparent
change in the frequency of any wave (e.g.,
its pitch if it is a sound wave) as the
observer and the source move toward or
away from each other.
Dose The amount of radiant energy ab-
sorbed by an irradiated object per unit mass.
Dose area product (DAP) The sum total
of air kerma over the exposed area of the
patient’s surface.
Dose commitment The total dose that
could ultimately be delivered from a given
intake of radionuclide that may remain in
the body for some time.
Dose length product (DLP) A measure
of the radiation dose for an entire CT
scan. It is mathematically equal to the
product of the volume CTDI and the scan
length. It has the units of mGy-cm.
Dose rate constant Also known as the
specific gamma ray constant “ G “, it is the
dose rate, usually specified in mSv/hr (rem/
hr in traditional units) at a specified distance from a source of activity given either
in MBq/hr or mCi/hr. For example, in SI
units the radioisotope Tc
99m
has a G value
approximately equal to: 2 (10)-5 mSv/hr per
MBq at one centimeter.
Double-strand break The ionization of a
DNA macromolecule that results in the
rupture or breakage of both of the two
sugar–phosphate chains of the DNA ladder-like molecular structure. This results
in splintering of a chromosome.
Doubling dose The radiation dose that
causes the number of spontaneous mutations occurring in a given generation to
increase to two times their original number.
Dual Energy X-Ray Absorptiometry
(DEXA, or DXA Scan) A noninvasive
x-ray procedure that can quantitively
predict the risk of bone fracture(s). For
this scan two different low-energy level
x-ray beams are directed toward bony
areas of interest and the ratio of their
relative degrees of absorption are compared with established normal ratio value
ranges.
E
Early somatic tissue reactions Reactions
in biologic tissues that are dependent on
the duration of time after the exposure to
ionizing radiation. They appear within
minutes, hours, days, or weeks of the time
of exposure. These reactions are precipitated by cell death.
Edema Swelling caused by excess fluid
trapped in body tissue.
Effective atomic number (Zeff) A com-
posite Z value for when multiple chemical
elements comprise a material.
Effective dose (EfD) Quantity that is
used for radiation protection purposes to
provide a measure of the overall risk of
exposure to humans from ionizing radiation. Effective dose takes into account the
dose from all types of ionizing radiation
(e.g., alpha, beta, gamma, x-ray) to various irradiated organs or tissues in the
human body (skin, gonadal tissue, thyroid). By including a specific weighting
factor for each of those parts of the body
mentioned, EfD takes into account the
chance or risk of each of those body parts
to develop a radiation-induced cancer (or
in the case of the reproductive organs, the
risk of genetic damage).
Effective dose (EfD) limit Concerns the
upper boundary dose of ionizing radiation
that results in a negligible risk of bodily
injury or hereditary damage. These limits
may be expressed for whole-body exposure, partial-body exposure, and exposure
of individual organs. Separate limits are set
for occupationally exposed individuals and
for the general public.
Effective dose (EfD) limiting system The
current method for assessing radiation exposure and associated risk of biologic damage to radiation workers and the general
public. It is a set of numeric dose limits that
are based on calculations of the various
risks of cancer and genetic effects to tissues
or organs exposed to radiation.
Effective CT dose (EfD) The product of
the scan DLP and the EfDLP.
EfDLP The normalized EfD associated
with a specific scan region of the body
(also known as the “k” factor).
Effective half-life (T
) The actual half-
eff
life of a radioactive material in a patient’s
body resulting from a combination of
natural decay and physical removal by
bodily functions.
Elastic scattering Most simply, this signi-
fies an interaction between two particles in
which the kinetic energy of each particle
remains unchanged but their directions of
motion are changed.
Elective examinations Nonurgent x-ray
examinations that can be scheduled at an
appropriate time to meet patient needs
and safety requirements.
Electric charge The physical property of
matter that causes it to experience a force
when placed in an electromagnetic field.
There are two types of electric charges:
positive and negative. Like charges repel,
and unlike charges attract.
Electrical potential difference (voltage
difference) A potential difference or
applied voltage is used to add kinetic energy to charged particles. The total energy
acquired by an electric charge when it is
placed in a potential difference of one volt
is equal to one electron-volt (eV) of energy. An electron in a potential difference
of 100,000 volts can acquire an energy of
100,000 eV or 100 keV.
Electrical potential energy The energy
attained by a charged particle as a result
of the voltage difference to which it is exposed. Electrical potential energy may be
measured in units of joules or electronvolts. When an x-ray tube uses an applied
voltage of 100,000 volts, it accelerates
electrons toward its anode with an energy
of 100 keV.
Electrocardiogram (ECG) A graphical
tracing or readout of the pulses or electrical activity with time that occur during the
heart cycle of contraction and relaxation.
ECG gated imaging An imaging method
whereby the CT scanner is electronically
linked to the patient’s real-time electrocardiogram. CT image acquisition and reconstruction can then be coordinated with the
quiescent heart phases associated with particular magnitude electrical pulses, thereby
removing or minimizing motion artifacts.
Electrolytes See Salts.
Electromagnetic radiation Radiation
composed of interacting, varying electric
and magnetic fields that propagate
through space at the speed of light. Examples include radio waves, microwaves,
visible light, ultraviolet rays, x-rays, and
gamma rays.
376
GLOSSARY
Electromagnetic spectrum The full
range of frequencies and wavelengths of
electromagnetic waves.
Electromagnetic wave Electric and mag-
netic fields that fluctuate rapidly as they
travel through space, including radio waves,
microwaves, visible light, and x-rays.
Electrometer A device used to measure
the quantity of electrical charge.
Electron capture A process wherein an
inner-shell electron is captured by one of
the nuclear protons, followed directly by
the two combining to produce a neutron.
Electrons Negatively charged fundamen-
tal particles.
Element A substance made up of atoms
that all have the same atomic number and
hence the same chemical properties.
Embryologic effects Damage to an or-
ganism that occurs as a result of exposure
to some agent (such as ionizing radiation)
during the embryonic stage of development. Also known as birth defects.
Endoplasmic reticulum A vast, irregular
network of tubules and vesicles spreading
and interconnecting in all directions
throughout the cytoplasm, thus enabling
the cell to intercommunicate throughout
the extranuclear environment and transfer
food from one part of the cell to another.
Energy The ability to do work.
Enhanced natural sources Natural
sources of ionizing radiation that grow
larger because of accidental or deliberate
human actions.
Entrance exposure Quantity of radia-
tion, specified in SI units (coulombs per
kilogram) or traditional units (roentgens), incident on an object.
Entrance skin exposure (ESE) X-ray ex-
posure to the skin of the patient.
Entrance skin exposure dose (ESEd)
rates Limited by federal standards of
general-purpose intensified fluoroscopic
units to a maximum of 88 mGya per minute. Measured at tabletop with the image
intensifier entrance surface at a prescribed
30 cm above the tabletop.
Environmental Protection Agency (EPA)
US government agency that facilitates the
development and enforcement of regulations pertaining to the control of radiation
in the environment. It directs federal
agencies, oversees the general area of environmental monitoring, and has authority
over specific areas such as determining the
action level for radon.
Enzymatic proteins Proteins that control
the cell’s various physiologic activities by
functioning as catalysts. Also known as
enzymes.
Epidemiology “Science that deals with
the incidence, distribution, and control of
disease in a population.”
Epidermis Outer layer of skin.
Epilation Loss of hair by various means.
The resultant effect, if permanent, is
known as alopecia.
Epithelial tissue A substance that lines
and covers body tissue; the cells that comprise this tissue are highly radiosensitive
because the body constantly regenerates
this tissue.
Equivalent dose (EqD) A radiation
quantity used for radiation protection
purposes when a person receives exposure
from various types of ionizing radiation.
This quantity attempts to numerically
specify the differences in transferred energy and therefore potential biologic
harm that are produced by different types
of radiation. EqD is the product of the
average absorbed dose in a tissue or organ
in the human body and its associated radiation weighting factor chosen for the
type of radiation in question. Equivalent
dose enables the calculation of the effective dose (EfD).
Erg A unit of energy and work that is equal
to 1027 joules.
Erythema Diffused redness over an area
of skin after irradiation.
Erythroblasts Red blood stem cells.
Erythrocytes Red blood cells without a
nucleus that contain the protein hemoglobin which, besides imparting the red
color to blood, facilitates the transport of
oxygen from the lungs to all body tissues
and cells as blood circulates.
Europium-activated barium fluoro-
halide The most commonly employed
photostimulable phosphor used in computed radiography imaging plates as the
image receptor.
Excess cancers Cancers that would not
have occurred in a population without
exposure to ionizing radiation.
Excitation The addition of energy to a
system, thereby transforming it usually
from a ground, or lowest energy, state to
an excited, or higher-energy state.
Exit, or image formation, radiation All
the x-ray photons that reach their destination (the image receptor) after passing
through the patient being radiographed;
previously known as remnant radiation.
Exposure The total electric charge of one
sign, either all pluses or all minuses, per
unit mass that x-ray and gamma ray photons with energies up to 3 million electron volts (MeV) generate in dry (i.e.,
nonhumid) air at standard temperature
and pressure (760 mmHg or 1 atmosphere at sea level and 22°C); the amount
of ionizing radiation that may strike an
object, such as the human body, when in
the vicinity of a radiation source. In the SI
system it can be measured in coulombs
per kilogram (C/kg) and in traditional
units as roentgens.
Exposure linearity Consistency in output
radiation intensity at any selected kVp
when x-ray generator settings are changed
from one milliamperage and time combination to another. Mathematically, it is
the ratio of the difference in mR/mAs
values between two successive x-ray unit
generator stations to the sum of those
mR/mAs values. It must be less than 0.1.
When changing from one mA station to a
neighboring mA station, the most that
linearity can vary is 10%.
Exposure rate dose Given in units of
mGya per hour; the same quantity as air
kerma rate.
Exposure reproducibility Consistency in
output in radiation intensity for identical
generator settings from one individual exposure to subsequent exposures. This
means that the x-ray unit must have the
ability to duplicate certain radiographic exposures for any given combination of peak
kilovolts (kVp), milliamperes (mA), and
time. A variance of 5% or less is acceptable.
Extremity dosimeter A device that mon-
itors the equivalent dose of radiation to
the hands.
Eye equivalent dose Radiation equiva-
lent dose to the lens of the eye at a tissue
depth of 0.3 cm.
F
Fallout Radiation produced as a conse-
quence of nuclear weapons testing.
Fats Compounds composed of long chains
of carbon, hydrogen, and oxygen, with the
ratio of hydrogen atoms to oxygen atoms
being much greater than 2:1; a rich energy
source. (See Lipids.)
Fatty acids Compounds formed when fat
combines with an acidic group of atoms